1. Field of the Invention
The present invention relates to an electric power steering device for a vehicle.
2. Prior Art
An electric power steering device for a vehicle is configured so that, when a driver handles a steering wheel, a controller of the power steering device detects steering torque caused by a steering shaft with a torque sensor and computes a command current value which is a control target of the motor current supplied to a motor which generates auxiliary steering force based on the detected steering torque. Also the controller detects a motor current actually flowing in the motor and feed it back to the command current value. The motor is driven by the controlled motor current value controlled so that the difference between the command current value and the detected motor current value is equal to zero and auxiliary steering force conforming to the steering torque is supplied to the steering system.
In an electric power steering wheel configured as stated above, whereas steering torque is not generated and essentially motor current is in the state of zero, a motor current detection signal is output due to the offset of a differential amplifier and the like constituting a motor current detector. When the motor current detection signal is fed back to the command current value, the motor is driven by the controlled motor current value based on the feedback and the auxiliary steering force output from the motor deviates to the extent corresponding to the offset. As a consequence, the disadvantage here is that, when a driver handles a steering wheel, the driver feels difference between steering to the left and steering to the right and the fluctuation of steering torque in the case of a brushless DC motor.
As a measure to overcome the disadvantage, it is proposed a means of outputting a detected motor current value as an offset correction value while steering torque is not generated and essentially motor current is in the state of not flowing and correcting the command current value with the offset correction value (refer to Japanese Patent Nos. 2914480 and 2847406).
FIG. 7 is an example of a block diagram showing the electronic control circuit 100 of an electric power steering device configured as stated above. In FIG. 7, a command current value computing unit 103 receives the steering torque T detected with a torque sensor 101 and the vehicle speed V detected with a vehicle speed sensor 102 and computes the command current value I which is the control target value of the motor current. The reference numeral 104 is an adder and it adds the command current value I to the detected motor current value ie corrected with an after-mentioned offset correction computing unit 109 and outputs the result as a controlled current value E.
A motor current detector 108 detects the motor current i actually flowing in a motor and the detected motor current value i is corrected with an offset correction value ΔI stored in the after-mentioned offset correction computing unit 109.
A drive controller 105 decides the duty ratio D of a pulse width modulation (PWM) signal to drive the motor based on a controlled current value E which has been inputted from adder 104. A motor drive circuit 106 drives the motor 107 based on the decided duty ratio D.
The offset correction computing unit 109 stores as an offset value ΔI which is detected motor current value i outputted from the motor current detector 108 in the state wherein a command current value I is equal to zero (0), namely motor current does not flow essentially. Thereafter the unit 109 corrects the detected motor current value i with the offset value ΔI and outputs the corrected detected motor current value ie to the adder 104.
By the above configuration, when a driver handles a steering wheel, since the detected motor current value i is corrected with the offset correction value ΔI and fed back, the controlled motor current value is corrected with the offset value and therefore it is possible to eliminate the disadvantageous situation wherein the driver feels difference between steering to the left and steering to the right and the fluctuation of steering torque in the case of a brushless DC motor.
In recent years, the size of a vehicle on which an electric power steering device is mounted is getting larger, thus the torque of a motor required in the electric power steering device is increasing, and the motor current and the output of the motor are also increasing. However, in a microcomputer (CPU) constituting the electronic control circuit of the electric power steering device, the resolution of an A/D converter to digitally convert a detected motor current value is still 10 bits and therefore the resolution becomes insufficient when the motor current increases. Further, since the torque constant increases, the torque per bit used in the CPU also increases.
As a result, the variation of the torque caused by the deviation of one least significant bit in the ten-bit resolution, namely torque ripples, comes close to the extent that a driver handling a steering wheel can feel the variation and thus even the error of about one or two bits that has not heretofore been a problem has not been ignored.
In particular, since the offset error of a motor current detector, even with the error of about one to three bits, directly affects torque ripples, an electric power steering device of a high output may possibly generate vibration and noise.
In addition, the offset error of a motor current detector must be corrected for each electric power steering device since the variation of the thermal characteristics of circuit elements is large. Nevertheless, it is not realistic to correct the offset error over the full temperature range (−40° C. to 120° C.) at the time of the production from the view point of the workload. Another problem is that the change of offset error caused by aging must also be taken into consideration.